This invention concerns a system for detecting at a distance persons or objects by means of ultra high frequency energy. In particular the invention concerns such a system intended to seek out avalanche victims.
From U.S. Pat. No. 4,331,957 it is known to fashion a system of the type indicated above by means of a passive transponder element intended to be carried by a person and a combined transmitter-receiver transmitting at a given ultra-high frequency f.sub.o and tuned to a reception frequency 2f.sub.o, the energy radiated by the transmitter being partially returned by the transponder with a frequency double that of the transmitting frequency. In such a system it is naturally convenient to reject in the emission spectrum of the transmitter the component 2f.sub.o which explains the utilization of the band of ultra-high frequencies in which such a rejection belongs completely to the domain of the possible. In practice one may thus employ for the frequency f.sub.o a value on the order of Gigahertz.
The transponder described in the previously mentioned patent is obtained in conformity with FIG. 1 of the attached drawings. A plate A in dielectric material is covered over its entire surface by a protective leaf B and between this plate and the leaf there is sandwiched an antenna C. The latter is realized by means of a metallic leaf and exhibits two radiating elements D.sub.1 and D.sub.2 connected together by a non-linear passive component E, more particularly a diode having a quadratic current-voltage characteristic. Moreover, the two radiating elements are further connected by a conductor F. It will be likewise noted that each radiating element D.sub.1 and D.sub.2 is narrowed from an end zone of length l to form on the opposite end a connection point to the diode.
There is shown on FIG. 2 the equivalent schematic of the transponder of FIG. 1. In order that the antenna may capture a maximum of energy at frequency f.sub.o, it is preferable to give it at least approximately a length 1/2 .lambda..sub.o, which is the half wave length of the frequency which it receives from the transmitter. Furthermore, each radiating element D.sub.1 and D.sub.2 has a length 1/4 .lambda..sub.o in order to be able to emit at the frequency 2f.sub.o at which the energy captured by the antenna is retransmitted thanks to diode E. From here one may assimilate the antenna in its energy reception configuration to a generator G producing a voltage U.sub.g at frequency f.sub.o and giving it out via a capacity C.sub.j, which is that of the diode E in a utilizing circuit formed by radiating element D.sub.1 and D.sub.2, with a certain loss in energy which is symbolized here by resistance R.sub.p. The circuit output voltage U.sub.1 will thus be lower than that of the generator G.
In order to obtain a better yield of the transformation of the energy at frequency f.sub.o to energy at frequency 2f.sub.o, the U.S. patent previously mentioned provides coupling together the two radiating elements by conductor F. The latter constitutes a conductive path for the direct current and brings about a certain bias of diode E which may then be employed with an operating point situated at a higher level in its current-voltage characteristic.
To give a concrete idea of the arrangement it is useful to note that the length of the antenna may be about 125 mm, its width being 22 mm, for instance, these dimensions being necessary to obtain with a certain power of transmission from the search apparatus a reflection of energy detectable by the receiver thereof and capable of being exploited for the seeking out of persons.
This invention has as purpose to provide a transponder conceived according to the principle which has just been described but which either generates a retransmission of power at a higher level for dimensions comparable to that of the transponder of the prior art, or produces with smaller dimensions a retransmission power comparable to that generated by the prior art transponder.